Light-Emitting Device and Camera

ABSTRACT

To provide a light-emitting device whose amount of light can be adjusted, or the like. The amount of light emitted from the light-emitting device can be adjusted by controlling the magnitude of the constant current pulse by a control signal. Specifically, the light-emitting device includes a constant current supply configured to be supplied with a control signal and a control pulse signal and configured to supply a constant current pulse; a control device configured to supply the control signal; a driver circuit configured to supply the control pulse signal; and a light-emitting panel configured to be supplied with the constant current pulse. The control signal is a signal for controlling the magnitude of the constant current pulse. The light-emitting panel includes a light-emitting element. The current density of the light-emitting element is greater than or equal to 10 mA/cm 2  and less than or equal to 1000 mA/cm 2 .

This application is a divisional of copending U.S. application Ser. No.15/958,305, filed on Apr. 20, 2018 which is a continuation of U.S.application Ser. No. 15/275,684, filed on Sep. 26, 2016 (now U.S. Pat.No. 9,961,726 issued May 1, 2018) which is a continuation of U.S.application Ser. No. 15/053,401, filed on Feb. 25, 2016 (now U.S. Pat.No. 9,462,646 issued Oct. 4, 2016) which is a continuation of U.S.application Ser. No. 14/315,675, filed on Jun. 26, 2014 (now U.S. Pat.No. 9,277,627 issued Mar. 1, 2016), which are all incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a light-emittingdevice, an electronic device, and a method for driving thelight-emitting device or the electronic device. In particular, oneembodiment of the present invention relates to a light-emitting deviceand an electronic device using an organic electroluminescence(hereinafter also referred to as EL) phenomenon, and a method fordriving the light-emitting device or the electronic device.

2. Description of the Related Art

Research and development have been extensively conducted onlight-emitting elements using organic electroluminescence (EL) (alsoreferred to as organic EL elements). In a basic structure of an organicEL element, a layer containing a light-emitting organic compound (alsoreferred to as an EL layer) is provided between a pair of electrodes. Byapplying voltage to this element, light emission from the light-emittingorganic compound can be obtained.

Since an organic EL element can be formed in a film form, an elementwith a large area can be easily formed. Thus, organic EL elements alsohave great potential as planar light sources which can be applied tolighting devices and the like.

For example, a lighting device including an organic EL element isdisclosed in Patent Document 1.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2009-130132

SUMMARY OF THE INVENTION

Cameras are equipped with flashes for taking photographs in a darkplace.

Smaller and/or lighter cameras are demanded for easy portability.

However, as the size of a flash decreases, the shape of itslight-emitting portion becomes more linear or point-like. Light from alight source travels in a straight line; therefore, a smaller lightsource casts a sharper shadow of an object. Thus, when a photograph of ahuman face is taken in a dark place using a flash, for example, a shadowof a nose might be cast on a cheek.

In addition, when the intensity of a flash is excessively high, aportion having different brightness in reality becomes uniformly whitein a photograph (blown-out highlights) in some cases. In contrast, whenthe intensity of a flash is too low, a dark portion of a photographbecomes uniformly black (blocked up shadows) in some cases. For thesereasons, a flash whose amount of light can be adjusted depending on anenvironment or a subject has been required.

For example, the amount of light of a light-emitting diode using aninorganic material can be adjusted, but the light-emitting diode is apoint light source. When a point light source, a surface light source,and a line light source emit the same amount of light, it is necessarythat the point light source have a larger amount of light per unit areaor a longer emission time than the surface light source and the linelight source. Since the light-emitting diode generates heat when itemits light, a prolonged light emission or a light emission with a largeamount of current increases the amount of heat generation of thelight-emitting diode; thus, a decrease in lifetime or breakage of anelement might occur. In terms of reliability, a current greater than orequal to 1.0 A or 1.5 A is difficult to make flow even in ahigh-luminance light-emitting diode.

Therefore, an object of one embodiment of the present invention is toprovide a light-emitting device whose amount of light can be adjusted.Another object of one embodiment of the present invention is to providea highly reliable light-emitting device. Another object of oneembodiment of the present invention is to provide a light-emittingdevice with low power consumption. Another object of one embodiment ofthe present invention is to provide a light-emitting device that is lesslikely to produce a shadow. Another object of one embodiment of thepresent invention is to reduce the size or weight of a light-emittingdevice.

In one embodiment of the present invention, there is no need to achieveall the objects.

A light-emitting device of one embodiment of the present inventionincludes a constant current supply configured to be supplied with acontrol signal and a control pulse signal and configured to supply aconstant current pulse; a control device configured to supply thecontrol signal; a driver circuit including a start switch and configuredto supply the control pulse signal in accordance with switchingoperation of the start switch; and a light-emitting panel configured tobe supplied with the constant current pulse. The control signal is asignal for controlling the magnitude of the constant current pulse. Thelight-emitting panel includes a light-emitting element. The currentdensity of the light-emitting element is greater than or equal to 10mA/cm² and less than or equal to 1000 mA/cm².

In the above light-emitting device, for example, the driver circuit maysupply the control pulse signal so that the constant current supplysupplies a constant current with a half width of more than or equal to 1millisecond and less than or equal to 1000 milliseconds.

A light-emitting device of one embodiment of the present inventionincludes a switching circuit configured to be supplied with a constantcurrent and a control pulse signal and configured to supply a constantcurrent pulse; a constant current supply configured to be supplied witha control signal and configured to supply the constant current; acontrol device configured to supply the control signal; a driver circuitincluding a start switch and configured to supply the control pulsesignal in accordance with switching operation of the start switch; and alight-emitting panel configured to be supplied with the constant currentpulse. The control signal is a signal for controlling the magnitude ofthe constant current pulse. The light-emitting panel includes alight-emitting element. The current density of the light-emittingelement is greater than or equal to 10 mA/cm² and less than or equal to1000 mA/cm².

In the above light-emitting device, for example, the driver circuit maysupply the control pulse signal so that the switching circuit supplies aconstant current with a half width of more than or equal to 1millisecond and less than or equal to 1000 milliseconds.

The above light-emitting device may further include an optical sensorthat supplies a first detection signal in accordance with the detectedamount of light. In that case, the control device includes an arithmeticunit and to which the first detection signal is supplied. In thearithmetic unit, an arithmetic operation is performed using the firstdetection signal. The control device supplies a control signal so thatthe constant current supply supplies a constant current in accordancewith the result of the arithmetic operation.

The above light-emitting device may further include a distance sensorthat supplies a second detection signal in accordance with the detecteddistance. In that case, the control device includes an arithmetic unitand to which the second detection signal is supplied. In the arithmeticunit, an arithmetic operation is performed using the second detectionsignal. The control device supplies a control signal so that theconstant current supply supplies a constant current in accordance withthe result of the arithmetic operation.

The above light-emitting device may include both the optical sensor andthe distance sensor. In that case, the control device includes anarithmetic unit and to which the first detection signal and the seconddetection signal are supplied. In the arithmetic unit, an arithmeticoperation is performed using the first detection signal and the seconddetection signal. The control device supplies a control signal so thatthe constant current supply supplies a constant current in accordancewith the result of the arithmetic operation.

In the above light-emitting device, the light-emitting panel may includea support substrate and a light-emitting element over the supportsubstrate, and the light-emitting element may include a first electrodeon a support substrate side, a second electrode overlapping with thefirst electrode, and an EL layer between the first electrode and thesecond electrode. In that case, the support substrate may be a flexiblesubstrate, and the light-emitting panel may have a curved surface.

In the above light-emitting device, the constant current supply mayfurther include an AC-DC converter configured to supply a direct currentand a DC-DC converter configured to be supplied with the direct currentand configured to supply a constant current.

Note that in this specification, the AC-DC converter refers to a devicewhich converts an alternating current into a direct current, and theDC-DC converter refers to a device which converts a voltage of a directcurrent from one level to another. The constant current power source mayinclude the DC-DC converter as well as a current sensor.

The above light-emitting device may include a battery for supplying afirst voltage, a first DC-DC converter configured to be supplied withthe first voltage and configured to supply a second voltage higher thanthe first voltage, a capacitor configured to be supplied with the secondvoltage, and a second DC-DC converter configured to be supplied with acharge from the capacitor and configured to supply a constant current.

Electronic devices, such as a camera and a digital still camera,including the light-emitting device with the above structure areembodiments of the present invention.

One embodiment of the present invention provides a light-emitting devicewhose amount of light can be adjusted, a highly reliable light-emittingdevice, a light-emitting device with low power consumption, or alight-emitting device which is less likely to produce a shadow. Oneembodiment of the present invention achieves a reduction in size and/orweight of a light-emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are block diagrams and timing charts for illustratinglight-emitting devices.

FIGS. 2A to 2C are a block diagram and timing charts for illustrating alight-emitting device.

FIGS. 3A and 3B illustrate a light-emitting panel.

FIGS. 4A and 4B illustrate light-emitting panels.

FIGS. 5A and 5B illustrate light-emitting panels.

FIGS. 6A to 6C illustrate light-emitting panels.

FIGS. 7A to 7D illustrate light-emitting elements.

FIGS. 8A to 8C illustrate electronic devices.

FIGS. 9A to 9C illustrate electronic devices.

FIG. 10 is a block diagram for illustrating a light-emitting device.

FIG. 11 is a graph showing luminance vs. voltage characteristics of alight-emitting panel in Example.

FIG. 12 is a graph showing an emission spectrum of a light-emittingpanel in Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription, and it will be easily understood by those skilled in theart that various changes and modifications can be made without departingfrom the spirit and scope of the present invention. Therefore, thepresent invention should not be construed as being limited to thedescription in the following embodiments. Note that in the structures ofthe invention described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals indifferent drawings, and description of such portions is not repeated.

Embodiment 1

In this embodiment, configurations of light-emitting devices of oneembodiment of the present invention will be described with reference toFIGS. 1A to 1D and FIGS. 2A to 2C.

In the light-emitting device of one embodiment of the present invention,the amount of light emitted from a light-emitting panel can be adjustedby a control signal supplied by a control device. The amount of lightmay be adjusted manually by a user of the light-emitting device orautomatically by the light-emitting device in accordance with thebrightness around the light-emitting device detected by an opticalsensor, a distance from the light-emitting device to an object (e.g., asubject for photography) detected by a distance sensor, and the like. Ina control device capable of arithmetic processing, an arithmeticoperation may be performed using the brightness, the distance, an imagetaken in advance, or the like, and the amount of light may be adjustedin accordance with results of the arithmetic operation.

A light-emitting device of one embodiment of the present inventionincludes a light-emitting panel that is a surface light source. Forexample, with the use of an organic EL element, a thin and large-areaelement can be formed easily. When a surface light source, a point lightsource, and a line light source emit the same amount of light, thesurface light source can have a smaller amount of light per unit area ora shorter emission time than the point light source and the line lightsource. In addition, the surface light source releases heat easilybecause of its large light-emitting area. Accordingly, deterioration dueto local heat generation of the light-emitting panel can be suppressed.Therefore, the light-emitting device of one embodiment of the presentinvention has high reliability and its amount of light can be adjustedin a wide range.

Configuration Example 1

A light-emitting device 100 illustrated in FIG. 1A includes alight-emitting panel 120, a driver circuit 130, a constant currentsupply 140 a, and a control device 150. FIG. 1D illustrates an exampleof a change over time of the current supplied by the DC-DC converter.For example, a current of 2 A can be supplied to the light-emittingpanel 120 for 50 milliseconds.

A control signal and a control pulse signal are supplied to the constantcurrent supply 140 a. The constant current supply 140 a can supply aconstant current pulse to the light-emitting panel 120.

The control device 150 can supply a control signal. The control signalis a signal for controlling the magnitude of a constant current pulse.By changing the magnitude of a constant current pulse, the amount oflight emitted from the light-emitting panel 120 can be adjusted.

For example, in the case where the control device 150 is supplied with asignal corresponding to the amount of light that is selected by a userof the light-emitting device, the control device 150 may supply acontrol signal in accordance with the signal. In the case where thecontrol device 150 is supplied with a detection signal from a variety ofsensors, the control device 150 may supply a control signal inaccordance with the detection signal. In the case where the controldevice 150 includes an arithmetic unit, the control device 150 mayperform an arithmetic operation using a signal supplied to the controldevice 150 and supply a control signal in accordance with the result ofthe arithmetic operation.

The driver circuit 130 includes a start switch 132 and can supply acontrol pulse signal. The driver circuit 130 supplies a control pulsesignal in accordance with switching operation of the start switch 132.For example, the driver circuit 130 supplies a control pulse signal sothat the constant current supply 140 a supplies a constant current witha half width of more than or equal to 1 millisecond and less than orequal to 1000 milliseconds. Since the control pulse signal is suppliedby the driver circuit 130 to the constant current supply 140 a, theconstant current supply 140 a can supply a pulsed constant current tothe light-emitting panel 120.

<<Light-Emitting Panel>>

The light-emitting panel 120 is a surface light source, and includes asupport substrate and a light-emitting element over the supportsubstrate. The number of light-emitting elements may be one or more. Themagnitude of constant current pulses supplied to a plurality oflight-emitting elements may be controlled by one control signal or maybe separately controlled by respective control signals.

As the light-emitting element, for example, an organic EL element can beused. The organic EL element includes a first electrode on the supportsubstrate side, a second electrode overlapping with the first electrode,and an EL layer between the first electrode and the second electrode.Note that the structure of the light-emitting panel 120 is described indetail in Embodiment 2, and the structure of the organic EL element isdescribed in detail in Embodiment 3.

The area of a light-emitting portion of the light-emitting panel 120 isgreater than or equal to 0.5 cm² and less than or equal to 1 m²,preferably greater than or equal to 5 cm² and less than or equal to 200cm², more preferably greater than or equal to 15 cm² and less than orequal to 100 cm².

The amount of light of the light-emitting panel 120 can be adjustedwithin a range where, for example, the current density of alight-emitting element is greater than or equal to 10 mA/cm² and lessthan or equal to 1000 mA/cm², preferably greater than or equal to 10mA/cm² and less than or equal to 1500 mA/cm², more preferably greaterthan or equal to 10 mA/cm² and less than 1700 mA/cm², still morepreferably greater than or equal to 1 mA/cm² and less than or equal to2000 mA/cm².

When an organic EL element is used in the light-emitting panel 120, theamount of light per unit area can be small because the area of alight-emitting portion of the light-emitting panel 120 can be easilyincreased. Thus, the amount of heat generation per unit area can bereduced. Therefore, a light-emitting device which has higher reliabilityand causes less deterioration of a light-emitting panel than alight-emitting device including a light-emitting diode or the like usingan inorganic material.

The light-emitting panel 120 can be thinner and lighter in the case ofusing an organic EL element than in the case of using a conventionalxenon lamp or the like. Heat generated by light emission is diffusedover a large area in the light-emitting panel 120 and is thereforereleased efficiently. Accordingly, heat accumulation in thelight-emitting panel 120 is suppressed, and thus, deterioration of thelight-emitting panel 120 is suppressed.

Since the light-emitting panel 120 is a surface light source, when thelight-emitting device of one embodiment of the present invention is usedfor a flash of a camera, a shadow is less likely to be produced on asubject for photography.

The light-emitting panel 120 can be configured to emit white light byusing a properly selected light-emitting organic compound. For example,a plurality of light-emitting organic compounds which emit light ofcomplementary colors can be used. Alternatively, three kinds oflight-emitting organic compounds which emit light of red, green, andblue can be used. Furthermore, different emission spectra can beselected from a variety of organic compounds. Accordingly, thelight-emitting device having excellent white balance can be obtained.

By using a light-emitting organic compound, an emission spectrum can bebroadened as compared with that of a light-emitting diode using aninorganic material. Light having a broad emission spectrum is close tonatural light and suitable for photography.

The light-emitting panel 120 may include a plurality of light-emittingelements exhibiting different colors. If a color or color temperature ofa flash of a camera can be changed, the reproducibility of a subject, anenvironment, an atmosphere, and the like when a photograph is taken canbe enhanced. In addition, a light-emitting device may include aplurality of light-emitting panels exhibiting different colors.

A flexible light-emitting panel including a flexible support substratecan be placed along a housing having a curved surface. In that case, alight-emitting device can be placed regardless of the design of ahousing. For example, a flash can be placed along a camera housinghaving a curved surface.

<<Control Device>>

In the case where the control device 150 includes an arithmetic unit, anarithmetic operation can be performed using a signal supplied to thecontrol device 150. The signal supplied to the control device 150 is,for example, a detection signal supplied by a variety of sensorsincluding an optical sensor or a distance sensor, a signal obtained byamplifying the detection signal by an amplifier, or a signal obtained byconverting the detection signal or the amplified signal from an analogsignal to a digital signal by a converter. The control device 150 mayinclude, for example, a processor such as a central processing unit(CPU) or a digital signal processor (DSP), or a memory such as a RAM ora ROM in which an arithmetic operation program is stored.

<<Constant Current Supply>>

The constant current supply 140 a includes an AC-DC converter configuredto supply a direct current and a DC-DC converter configured to besupplied with the direct current. The timing of supply of the constantcurrent from the DC-DC converter is controlled by a control pulsesignal. Thus, a waveform of a constant current pulse can be shaped.

<<Driver Circuit>>

The driver circuit 130 supplies a control pulse signal with apredetermined width. The predetermined width is, for example, more thanor equal to 1 millisecond and less than or equal to 1000 milliseconds,preferably more than or equal to 10 milliseconds and less than or equalto 100 milliseconds.

For example, the driver circuit 130 can be configured using the startswitch 132, a latch circuit, and a monostable multivibrator.

Specifically, the start switch 132 is used to supply a high or lowsignal to the latch circuit. The latch circuit supplies a triggersignal. The monostable multivibrator supplied with the trigger signalsupplies a rectangular wave with a predetermined width as the controlpulse signal.

Note that in Configuration example 1, the control pulse signal issupplied to the constant current supply 140 a and used to control theconstant current supply 140 a. However, the control pulse signal may besupplied to a switching circuit 110 as described later in Configurationexample 2.

Configuration Example 2

A light-emitting device 101 illustrated in FIG. 1B includes theswitching circuit 110, the light-emitting panel 120, the driver circuit130, the constant current supply 140 a, the control device 150, and anoptical sensor 160.

A constant current and a control pulse signal are supplied to theswitching circuit 110. The switching circuit 110 can supply a constantcurrent pulse. A control signal is supplied to the constant currentsupply 140 a. The constant current supply 140 a can supply a constantcurrent. The constant current pulse is supplied to the light-emittingpanel 120. The structure of the light-emitting panel 120 inConfiguration example 2 can be the same as that in Configuration example1.

The optical sensor 160 can supply a detection signal to the controldevice 150 in accordance with the detected amount of light. The controldevice 150 includes an arithmetic unit. In the arithmetic unit, anarithmetic operation is carried out using the detection signal. Thecontrol device 150 can supply a control signal to the constant currentsupply 140 a so that the constant current supply 140 a supplies aconstant current in accordance with the result of the arithmeticoperation. Since the control device 150 supplies a control signal to theconstant current supply 140 a, the constant current supply 140 a cansupply a constant current adjusted in accordance with the amount oflight detected by the optical sensor 160 to the light-emitting panel120.

The driver circuit 130 includes a start switch 132 and can supply acontrol pulse signal. The driver circuit 130 supplies a control pulsesignal in accordance with switching operation of the start switch 132.For example, the driver circuit 130 supplies a control pulse signal sothat the switching circuit 110 supplies a constant current with a halfwidth of more than or equal to 1 millisecond and less than or equal to1000 milliseconds. Since the control pulse signal is supplied by thedriver circuit 130 to the switching circuit 110, the switching circuit110 can supply a pulsed constant current to the light-emitting panel120.

Thus, the light-emitting device 101 can adjust the amount of lightemitted from the light-emitting panel 120, in accordance with the amountof light detected by the optical sensor 160. For example, the opticalsensor 160 detects the surrounding brightness and the control device 150carries out an arithmetic operation, whereby current supplied to thelight-emitting panel 120 can be adjusted so that the light-emittingpanel emits an optimum amount of light.

For example, in the case where the light-emitting device 101 is used fora flash of a camera, the amount of light emitted from the light-emittingdevice 101 may be controlled to become smaller the larger the amount oflight detected by the optical sensor 160 is (the brighter thesurroundings of a subject for photography are). Thus, blown-outhighlights or blocked up shadows of a photograph can be prevented.

In the case where the light-emitting device 101 is used for a light of avehicle or a bicycle, the light-emitting device 101 may be controlled toemit light when the amount of light detected by the optical sensor 160is smaller than or equal to a predetermined value. Furthermore, theamount of light emitted from the light-emitting device 101 may becontrolled to become smaller the smaller the detected amount is (thedarker the surroundings are and the more easily light emitted from thelight-emitting device 101 is perceived). Thus, emission of excessivelight can be prevented, so that power saving and a long lifetime of thelight-emitting device can be achieved.

<<Optical Sensor>>

The optical sensor 160 includes a photoelectric conversion element suchas a photodiode. The optical sensor 160 supplies a detection signal inaccordance with the detected amount of light to the control device 150.

<<Switching Circuit>>

The switching circuit 110 supplies the constant current pulse to thelight-emitting panel 120 while being supplied with the constant currentand the control pulse signal.

For example, the switching circuit 110 may include a power transistor ora power FET. Specifically, the switching circuit 110 can be configuredsuch that the control pulse signal is supplied to a gate of the powertransistor, the constant current is supplied to a first electrode of thepower transistor, and the light-emitting panel 120 is electricallyconnected to a second electrode of the power transistor.

FIG. 1C illustrates an example of a constant current supplied by theDC-DC converter in the light-emitting device 101 illustrated in FIG. 1B.FIG. 1D illustrates an example of a change over time of the currentsupplied by the switching circuit 110. For example, a current of 2 A canbe supplied to the light-emitting panel 120 for 50 milliseconds.

Configuration Example 3

A light-emitting device 102 illustrated in FIG. 2A is different from thelight-emitting device 101 illustrated in FIG. 1B in the structure of theconstant current supply and in that the light-emitting device 102includes a distance sensor 162 and a counter circuit 155. Note that thedescription in Configuration example 2 is referred to for the othercomponents of the light-emitting device 102 because they are the same asin the light-emitting device 101.

<<Distance Sensor>>

The distance sensor 162 supplies a detection signal in accordance with ameasured distance to the control device 150. As the distance sensor 162,a variety of sensors such as an ultrasonic distance sensor or a laserdistance sensor can be used.

The control device 150 carries out an arithmetic operation using adetection signal supplied by the optical sensor and a detection signalsupplied by the distance sensor. The control device 150 supplies acontrol signal to a constant current supply 140 b so that the constantcurrent supply 140 b supplies a constant current in accordance with theresult of the arithmetic operation. Since the control device 150supplies a control signal to the constant current supply 140 b, theconstant current supply 140 b can supply, to the switching circuit 110,a constant current adjusted in accordance with the amount of lightdetected by the optical sensor 160 or the distance measured by thedistance sensor 162.

Thus, the light-emitting device 102 can adjust the amount of lightemitted from the light-emitting panel 120 in accordance with the amountof light detected by the optical sensor 160 or the distance measured bythe distance sensor 162. For example, in the case where thelight-emitting device 102 is used for a flash of a camera, the opticalsensor 160 detects the brightness of the surroundings of a subject, thedistance sensor 162 measures the distance from the subject to thecamera, and the control device 150 carries out an arithmetic operation,whereby current supplied to the light-emitting panel 120 can be adjustedso that the light-emitting panel emits an optimum amount of light. Thus,blown-out highlights or blocked up shadows of a photograph can beprevented. In addition, emission of unnecessary light can be prevented,so that power saving and a long lifetime of the light-emitting devicecan be achieved.

<<Modification Example of Constant Current Supply>>

The constant current supply 140 b includes a battery for supplying afirst voltage, a first DC-DC converter configured to be supplied withthe first voltage and configured to supply a second voltage higher thanthe first voltage, a capacitor supplied with the second voltage, and asecond DC-DC converter supplied with a charge from the capacitor.

The first DC-DC converter steps up the voltage (the first voltage) ofthe battery to the second voltage and supplies the second voltage.

The capacitor is charged with the second voltage.

The second DC-DC converter is supplied with the charge stored in thecapacitor and supplies the constant current.

With this configuration, the second DC-DC converter can supply theconstant current while the capacitor is supplying the charge to thesecond DC-DC converter. Note that when the charge stored in thecapacitor is less than a predetermined level, the second DC-DC convertercannot supply the constant current.

An example of a change over time of the current supplied by the constantcurrent supply 140 b is illustrated in FIG. 2B.

The constant current supply 140 b can supply the constant current for aperiod at least longer than the width of the control pulse signal (e.g.,50 milliseconds) supplied by the driver circuit 130. When the currentflows through the switching circuit 110, the charge stored in thecapacitor is consumed; eventually, it becomes impossible for theconstant current supply 140 b to supply the constant current. As aresult, a current that is not a rectangular wave flows to thelight-emitting panel 120, whereby the light-emitting panel 120 emitslight at a useless luminance lower than a predetermined luminance andconsumes unnecessary power. The switching circuit 110 can prevent suchunnecessary power consumption by stopping the supply of current afterthe supply of current for a predetermined time. Note that an example ofa change over time of the current supplied by the switching circuit 110is illustrated in FIG. 2C.

In this manner, the constant current supply 140 b can supply theconstant current by using the battery. Accordingly, the light-emittingdevice 102 which can be easily carried around can be provided.

<<Counter Circuit>>

A counter circuit 155 counts the number of times the driver circuit 130supplies the control pulse signal. Thus, the number of times thelight-emitting panel 120 has emitted light can be known.

The luminance of the light-emitting panel 120 may be decreased dependingon the number of times the light-emitting panel 120 has emitted light.

The number of times counted by the counter circuit 155 may be fed backto the driver circuit 130 and the width of the control pulse signal maybe increased. Thus, the emission time of the light-emitting panel 120can be prolonged and the decrease in the luminance of the light-emittingpanel 120 can be compensated for.

Alternatively, the number of times counted by the counter circuit 155may be fed back to the constant current supply 140 b and the amount ofthe constant current supplied by the constant current supply 140 b maybe increased. Thus, the decrease in the luminance of the light-emittingpanel 120 can be compensated for.

One embodiment of the present invention is applied to a light-emittingdevice in the above manner, so that the amount of light emitted from thelight-emitting device can be adjusted by controlling a current value.Since the light-emitting device includes a light-emitting panel that isa surface light source, the light-emitting device is less likely toproduce a shadow when used as a flash. A light-emitting panel of thelight-emitting device is less likely to deteriorate when it emits alarge amount of light as compared with the case of using alight-emitting diode using an inorganic material. Thus, thelight-emitting device can have high reliability. The light-emittingdevice can be small and thin as compared with the case of using a xenonlamp or the like.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 2

In this embodiment, a structure of a light-emitting panel that can beused for the light-emitting device of one embodiment of the presentinvention will be described with reference to FIGS. 3A and 3B, FIGS. 4Aand 4B, and FIGS. 5A and 5B.

<<Configuration Example 1 of Light-Emitting Panel>>

FIG. 3A is a plan view illustrating a light-emitting panel of oneembodiment of the present invention, and FIG. 3B is a cross-sectionalview taken along dashed-dotted line A-B in FIG. 3A.

In the light-emitting panel illustrated in FIGS. 3A and 3B, thelight-emitting element 403 is provided in a space 415 surrounded by thesupport substrate 401, a sealing substrate 405, and a sealant 407. Thelight-emitting element 403 is an organic EL element having abottom-emission structure; specifically, the first electrode 421 whichtransmits visible light is provided over the support substrate 401, theEL layer 423 is provided over the first electrode 421, and the secondelectrode 425 which reflects visible light is provided over the EL layer423.

The structure of the light-emitting element used in one embodiment ofthe present invention is not limited to the bottom-emission structureand may be a top-emission structure, for example.

A first terminal 409 a is electrically connected to an auxiliary wiring417 and the first electrode 421. An insulating layer 419 is providedover the first electrode 421 in a region which overlaps with theauxiliary wiring 417. The first terminal 409 a is electrically insulatedfrom the second electrode 425 by the insulating layer 419. A secondterminal 409 b is electrically connected to the second electrode 425.Note that although the first electrode 421 is formed over the auxiliarywiring 417 in this embodiment, the auxiliary wiring 417 may be formedover the first electrode 421.

A light extraction structure 411 a is preferably provided at theinterface between the support substrate 401 and the atmosphere. Whenprovided at the interface between the support substrate 401 and theatmosphere, the light extraction structure 411 a can reduce light whichcannot be extracted to the atmosphere due to total reflection, resultingin increased light extraction efficiency of the light-emitting panel.

In addition, a light extraction structure 411 b is preferably providedbetween the light-emitting element 403 and the support substrate 401.When the light extraction structure 411 b has unevenness, aplanarization layer 413 is preferably provided between the lightextraction structure 411 b and the first electrode 421. Accordingly, thefirst electrode 421 can be a flat film, and generation of leakagecurrent in the EL layer 423 due to the unevenness of the first electrode421 can be prevented. Further, because of the light extraction structure411 b at the interface between the planarization layer 413 and thesupport substrate 401, light which cannot be extracted to the atmospheredue to total reflection can be reduced, so that the light extractionefficiency of the light-emitting panel can be increased.

As a material of the light extraction structure 411 a and the lightextraction structure 411 b, a resin can be used, for example.Alternatively, for the light extraction structure 411 a and the lightextraction structure 411 b, a hemispherical lens, a micro lens array, afilm provided with an uneven surface structure, a light diffusing film,or the like can be used. For example, the light extraction structure 411a and the light extraction structure 411 b can be formed by attachingthe lens or the film to the support substrate 401 with an adhesive orthe like which has substantially the same refractive index as thesupport substrate 401 or the lens or the film.

The surface of the planarization layer 413 which is in contact with thefirst electrode 421 is flatter than the surface of the planarizationlayer 413 which is in contact with the light extraction structure 411 b.As a material of the planarization layer 413, a material with alight-transmitting property and a high refractive index (e.g., glass, aresin, or a liquid substance such as a refractive index liquid) can beused.

Note that the light-emitting panel of one embodiment of the presentinvention is not necessarily provided with any light extractionstructure. In that case, the second electrode which reflects visiblelight can be used as a mirror, which is preferable.

<<Configuration Example 2 of Light-Emitting Panel>>

FIG. 4A is a plan view illustrating a light-emitting panel of oneembodiment of the present invention, and FIGS. 5A and 5B are each across-sectional view taken along dashed-dotted line X1-Y1 in FIG. 4A.

In the light-emitting panel illustrated in FIG. 5A, a light-emittingelement 1250 is provided over a support substrate 1220 with aninsulating film 1224 provided therebetween. An auxiliary wiring 1206 isprovided over the insulating film 1224 and is electrically connected toa first electrode 1201. The auxiliary wiring 1206 is partly exposed andfunctions as a terminal. An end portion of the first electrode 1201 andan end portion of a conductive layer 1210 are covered with a partitionwall 1205. Further, the partition wall 1205 is provided to cover theauxiliary wiring 1206 with the first electrode 1201 providedtherebetween. The light-emitting element 1250 is sealed with the supportsubstrate 1220, a sealing substrate 1228, and a sealant 1227. A lightextraction structure 1209 is attached to the surface of the supportsubstrate 1220. A flexible light-emitting panel can be obtained by usingflexible substrates as the support substrate 1220 and the sealingsubstrate 1228.

The light-emitting element 1250 is an organic EL element having abottom-emission structure; specifically, the first electrode 1201 whichtransmits visible light is provided over the support substrate 1220, anEL layer 1202 is provided over the first electrode 1201, and a secondelectrode 1203 which reflects visible light is provided over the ELlayer 1202.

In the light-emitting panel illustrated in FIG. 5B, a support substrate1229 having a light extraction structure is provided instead of thesupport substrate 1220 and the light extraction structure 1209 of thelight-emitting panel illustrated in FIG. 5A. The support substrate 1229has both a function as a support and a function of improving the lightextraction efficiency of the light-emitting panel.

As methods for forming a light-emitting element over a flexiblesubstrate in the case of manufacturing a flexible light-emitting panel,there are methods such as a first method in which the light-emittingelement is directly formed over a flexible substrate, and a secondmethod in which the light-emitting element is formed over a highlyheat-resistant substrate (hereinafter referred to as a formationsubstrate) which is different from a flexible substrate and thelight-emitting element is then separated from the formation substrateand transferred to the flexible substrate.

In the case of using a substrate that is resistant to heat applied inthe process of forming the light-emitting element, such as a glasssubstrate thin enough to have flexibility, the first method ispreferably used because the process can be simplified.

When the second method is used, an insulating film with low waterpermeability or the like which is formed over a formation substrate athigh temperature can be transferred to a flexible substrate. Therefore,even when an organic resin with high water permeability and low heatresistance or the like is used as a material of the flexible substrate,a flexible light-emitting panel with high reliability can bemanufactured.

<<Configuration Example 3 of Light-Emitting Panel>>

FIG. 4B is a plan view of a light-emitting panel of one embodiment ofthe present invention. FIGS. 6A and 6B are cross-sectional views takenalong the dashed-dotted line X2-Y2 in FIG. 4B, which illustrate examplesof cross-sectional structures. FIG. 6C is a cross-sectional view takenalong the dashed-dotted line X3-Y3 in FIG. 4B.

The light-emitting panel illustrated in FIGS. 6A to 6C is different fromthe light-emitting panel described in Configuration example 2 in thatthe light-emitting panel illustrated in FIGS. 6A to 6C includes anopening. Here, only different components are described in detail, andthe description of the light-emitting panel of Configuration example 2is referred to for the common components.

As illustrated in FIGS. 6A and 6B, the light-emitting panel preferablyincludes a sealant 1226 in the opening to prevent an electrode or an ELlayer from being exposed. Specifically, an opening is formed in thelight-emitting panel, and then the sealant 1226 is formed to at leastcover an exposed electrode and an exposed EL layer. The sealant 1226 maybe the same material as or a different material from the sealant 1227.

FIG. 6A illustrates an example of an opening formed in a region wherethe partition wall 1205 is not provided. FIG. 6B illustrates an exampleof an opening formed in a region where the partition wall 1205 isprovided.

A light-emitting panel is manufactured in the above manner, and a cameralens is provided to overlap with the opening, whereby a light-emittingportion can be provided around the camera lens. The light-emittingportion can be used as a flash of a camera.

Note that a light extraction structure may be provided on a surface ofthe substrate.

<<Materials of Light-Emitting Panel>>

Examples of materials that can be used for the light-emitting panel ofone embodiment of the present invention are described.

[Substrate]

The substrate through which light emitted from the light-emittingelement is extracted is formed using a material that transmits thelight. For example, a material such as glass, quartz, ceramics,sapphire, or an organic resin can be used.

The weight and thickness of the light-emitting panel can be decreased byusing a thin substrate. Furthermore, a flexible light-emitting panel canbe obtained by using a substrate that is thin enough to haveflexibility. The flexible light-emitting panel can be stored in a foldedstate when not in use. The flexible light-emitting panel can be used asa lighting device which emits flash light over a large area, as analternative to a board reflector in a photography studio. Alternatively,a foldable lighting device can be provided.

Examples of glass include alkali-free glass, barium borosilicate glass,aluminoborosilicate glass, and the like.

Examples of materials having flexibility and a light-transmittingproperty with respect to visible light include glass that is thin enoughto have flexibility, polyester resins such as polyethylene terephthalate(PET) and polyethylene naphthalate (PEN), a polyacrylonitrile resin, apolyimide resin, a polymethyl methacrylate resin, a polycarbonate (PC)resin, a polyethersulfone (PES) resin, a polyamide resin, a cycloolefinresin, a polystyrene resin, a polyamide imide resin, and a polyvinylchloride resin. In particular, a material whose thermal expansioncoefficient is low is preferred, and for example, a polyamide imideresin, a polyimide resin, or PET can be suitably used. A substrate inwhich a glass fiber is impregnated with an organic resin or a substratewhose thermal expansion coefficient is reduced by mixing an organicresin with an inorganic filler can also be used. A substrate using sucha material is lightweight, and thus a light-emitting panel using thissubstrate can also be lightweight.

Since the substrate through which light is not extracted does not need alight-transmitting property, a metal substrate using a metal material oran alloy material or the like can be used as well as the above-mentionedsubstrates. A metal material and an alloy material, which have highthermal conductivity, are preferred because they can easily conduct heatto the whole substrate and accordingly can prevent a local temperaturerise in the light-emitting panel. To obtain flexibility or bendability,the thickness of a metal substrate is preferably greater than or equalto 10 μm and less than or equal to 200 μm, more preferably greater thanor equal to 20 μm and less than or equal to 50 μm.

Although there is no particular limitation on a material of the metalsubstrate, it is preferable to use, for example, aluminum, copper,nickel, a metal alloy such as an aluminum alloy or stainless steel.

It is possible to use a substrate subjected to insulation treatment insuch a manner that a surface of a conductive substrate is oxidized or aninsulating film is formed on the surface. An insulating film may beformed by, for example, a coating method such as a spin-coating methodor a dipping method, an electrodeposition method, an evaporation method,or a sputtering method. An oxide film may be formed on the substratesurface by exposure to or heating in an oxygen atmosphere or by ananodic oxidation method or the like.

The flexible substrate may have a stacked structure of a layer of any ofthe above-mentioned materials and a hard coat layer (e.g., a siliconnitride layer) which protects a surface of the light-emitting panel fromdamage or the like, a layer (e.g., an aramid resin layer) which candisperse pressure, or the like. Furthermore, to suppress a decrease inthe lifetime of the light-emitting element due to moisture and the like,an insulating film with low water permeability may be provided. Forexample, a film containing nitrogen and silicon (e.g., a silicon nitridefilm, a silicon oxynitride film) or a film containing nitrogen andaluminum (e.g., an aluminum nitride film) may be provided.

The substrate may be formed by stacking a plurality of layers. When aglass layer is used, a barrier property against water and oxygen can beimproved and thus a reliable light-emitting panel can be provided.

A substrate in which a glass layer, an adhesive layer, and an organicresin layer are stacked from the side closer to a light-emitting elementcan be used. The thickness of the glass layer is greater than or equalto 20 μm and less than or equal to 200 μm, preferably greater than orequal to 25 μm and less than or equal to 100 μm. With such a thickness,the glass layer can have both a high barrier property against water andoxygen and a flexibility. The thickness of the organic resin layer isgreater than or equal to 10 μm and less than or equal to 200 μm,preferably greater than or equal to 20 μm and less than or equal to 50μm. With such an organic resin layer provided on an outer side of theglass layer, breakage or a crack of the glass layer can be inhibited,resulting in increased mechanical strength. With the substrate thatincludes such a composite material of a glass material and an organicresin, a highly reliable and flexible light-emitting panel can beprovided.

[Insulating Film]

An insulating film may be provided between the support substrate and thelight-emitting element. As the insulating film, an inorganic insulatingfilm such as a silicon oxide film, a silicon nitride film, a siliconoxynitride film, or a silicon nitride oxide film can be used. In orderto suppress the entry of moisture or the like into the light-emittingelement, an insulating film with low water permeability such as asilicon oxide film, a silicon nitride film, or an aluminum oxide film isparticularly preferable. For a similar purpose and with a similarmaterial, an insulating film covering the light-emitting element may beprovided.

[Partition Wall]

For the partition wall, an organic resin or an inorganic insulatingmaterial can be used. As the organic resin, for example, a polyimideresin, a polyamide resin, an acrylic resin, a siloxane resin, an epoxyresin, or a phenol resin can be used. As the inorganic insulatingmaterial, silicon oxide, silicon oxynitride, or the like can be used. Inparticular, a photosensitive resin is preferably used for easy formationof the partition wall.

There is no particular limitation on the method for forming thepartition wall. A photolithography method, a sputtering method, anevaporation method, a droplet discharging method (e.g., an inkjetmethod), a printing method (e.g., a screen printing method or an offsetprinting method), or the like can be used.

[Auxiliary Wiring]

The auxiliary wiring is not necessarily provided; however, the auxiliarywiring is preferably provided because voltage drop due to the resistanceof an electrode can be prevented.

For the auxiliary wiring, a single layer or a stacked layer using amaterial selected from copper (Cu), titanium (Ti), tantalum (Ta),tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), scandium(Sc), or nickel (Ni) or an alloy material including any of thesematerials as its main component is used. Aluminum can also be used as amaterial of the auxiliary wiring. In the case where aluminum is indirect contact with a transparent conductive oxide material, aluminummight corrode. For this reason, it is preferable that the auxiliarywiring have a stacked-layer structure in which aluminum be used for alayer that is not in contact with ITO or the like. The thickness of theauxiliary wiring can be greater than or equal to 0.1 μm and less than orequal to 3 μm, preferably greater than or equal to 0.1 μm and less thanor equal to 0.5

[Sealant]

A method for sealing the light-emitting panel is not limited, and eithersolid sealing or hollow sealing can be employed. For example, a glassmaterial such as a glass frit, or a resin material such as a resin thatis curable at room temperature (e.g., a two-component-mixture-typeresin), a light curable resin, or a heat-curable resin can be used. Thelight-emitting panel may be filled with an inert gas such as nitrogen orargon, or resin such as a polyvinyl chloride (PVC) resin, an acrylicresin, a polyimide resin, an epoxy resin, a silicone resin, a polyvinylbutyral (PVB) resin, or an ethylene vinyl acetate (EVA) resin. Further,a drying agent may be contained in the resin.

[Light Extraction Structure]

For the light extraction structure, a hemispherical lens, a micro lensarray, a film provided with an uneven surface structure, a lightdiffusing film, or the like can be used. For example, the lightextraction structure can be formed by attaching the lens or film to thesubstrate with an adhesive or the like which has substantially the samerefractive index as the substrate or the lens or film.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 3

In this embodiment, light-emitting elements which can be used in thelight-emitting device of one embodiment of the present invention will bedescribed with reference to FIGS. 7A to 7D.

<Structural Example of Light-Emitting Element>

A light-emitting element illustrated in FIG. 7A includes an EL layer 203between a first electrode 201 and a second electrode 205. In thisembodiment, the first electrode 201 serves as an anode, and the secondelectrode 205 serves as a cathode.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between the first electrode 201 and the secondelectrode 205, holes are injected to the EL layer 203 from the firstelectrode 201 side and electrons are injected to the EL layer 203 fromthe second electrode 205 side. The injected electrons and holesrecombine in the EL layer 203 and a light-emitting substance containedin the EL layer 203 emits light.

The EL layer 203 includes at least a light-emitting layer 303 containinga light-emitting substance.

In addition to the light-emitting layer, the EL layer 203 may furtherinclude one or more layers containing any of a substance with a highhole-injection property, a substance with a high hole-transportproperty, a substance with a high electron-transport property, asubstance with a high electron-injection property, a substance with abipolar property (a substance with a high electron-transport propertyand a high hole-transport property), and the like. For the EL layer 203,either a low molecular compound or a high molecular compound can beused, and an inorganic compound may also be used.

A light-emitting element illustrated in FIG. 7B includes the EL layer203 between the first electrode 201 and the second electrode 205, and inthe EL layer 203, a hole-injection layer 301, a hole-transport layer302, the light-emitting layer 303, an electron-transport layer 304, andan electron-injection layer 305 are stacked in that order from the firstelectrode 201 side.

As in light-emitting elements illustrated in FIGS. 7C and 7D, aplurality of EL layers may be stacked between the first electrode 201and the second electrode 205. In this case, an intermediate layer 207 ispreferably provided between the stacked EL layers. The intermediatelayer 207 includes at least a charge-generation region.

For example, the light-emitting element illustrated in FIG. 7C includesthe intermediate layer 207 between a first EL layer 203 a and a secondEL layer 203 b. The light-emitting element illustrated in FIG. 7Dincludes n EL layers (n is a natural number of 2 or more), and theintermediate layers 207 between the EL layers.

The following shows behaviors of electrons and holes in the intermediatelayer 207 between the EL layer 203(m) and the EL layer 203(m+1). When avoltage higher than the threshold voltage of the light-emitting elementis applied between the first electrode 201 and the second electrode 205,holes and electrons are generated in the intermediate layer 207, and theholes move into the EL layer 203(m+1) provided on the second electrode205 side and the electrons move into the EL layer 203(m) provided on thefirst electrode 201 side. The holes injected into the EL layer 203(m+1)recombine with the electrons injected from the second electrode 205side, so that a light-emitting substance contained in the EL layer203(m+1) emits light. Further, the electrons injected into the EL layer203(m) recombine with the holes injected from the first electrode 201side, so that a light-emitting substance contained in the EL layer203(m) emits light. Thus, the holes and electrons generated in theintermediate layer 207 cause light emission in the respective EL layers.

Note that the EL layers can be provided in contact with each other withno intermediate layer provided therebetween when these EL layers allowthe same structure as the intermediate layer to be formed therebetween.For example, when the charge-generation region is formed over onesurface of an EL layer, another EL layer can be provided in contact withthe surface.

Further, when emission colors of EL layers are made different, lightemission of a desired color can be provided from the light-emittingelement as a whole. For example, in a light-emitting element having twoEL layers, when the emission colors of first and second EL layers arecomplementary, the light-emitting element can emit white light as awhole. This can be applied to a light-emitting element having three ormore EL layers.

<<Material of Light-Emitting Element>>

Examples of materials which can be used for the layers are describedbelow. Note that each layer is not limited to a single layer, and may bea stack including two or more layers.

<Anode>

The electrode serving as the anode (the first electrode 201) can beformed using one or more kinds of conductive metals, alloys, conductivecompounds, and the like. In particular, it is preferable to use amaterial with a high work function (4.0 eV or more). Examples includeindium tin oxide (ITO), indium tin oxide containing silicon or siliconoxide, indium zinc oxide, indium oxide containing tungsten oxide andzinc oxide, graphene, gold, platinum, nickel, tungsten, chromium,molybdenum, iron, cobalt, copper, palladium, titanium, and a nitride ofa metal material (e.g., titanium nitride).

When the anode is in contact with the charge-generation region, any of avariety of conductive materials can be used regardless of their workfunctions; for example, aluminum, silver, an alloy containing aluminum,or the like can be used.

<Cathode>

The electrode serving as the cathode (the second electrode 205) can beformed using one or more kinds of conductive metals, alloys, conductivecompounds, and the like. In particular, it is preferable to use amaterial with a low work function (3.8 eV or less). Examples includealuminum, silver, an element belonging to Group 1 or 2 of the periodictable (e.g., an alkali metal such as lithium or cesium, an alkalineearth metal such as calcium or strontium, or magnesium), an alloycontaining any of these elements (e.g., Mg—Ag or Al—Li), a rare earthmetal such as europium or ytterbium, and an alloy containing any ofthese rare earth metals.

Note that when the cathode is in contact with the charge-generationregion, a variety of conductive materials can be used regardless of itswork function. For example, ITO, indium tin oxide containing silicon orsilicon oxide, or the like can be used.

The electrodes may be formed separately by a vacuum evaporation methodor a sputtering method. Alternatively, when a silver paste or the likeis used, a coating method or an inkjet method may be used.

<Hole-Injection Layer 301>

The hole-injection layer 301 contains a substance with a highhole-injection property.

Examples of the substance with a high hole-injection property includemetal oxides such as molybdenum oxide, vanadium oxide, ruthenium oxide,tungsten oxide, and manganese oxide; and phthalocyanine-based compoundssuch as phthalocyanine (abbreviation: H₂Pc) and copper(II)phthalocyanine (abbreviation: CuPc).

Other examples of the substance with a high hole-injection propertyinclude high molecular compounds such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA);and high molecular compounds to which acid is added such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid)(abbreviation: PEDOT/PSS).

The hole-injection layer 301 may serve as the charge-generation region.When the hole-injection layer 301 in contact with the anode serves asthe charge-generation region, any of a variety of conductive materialscan be used for the anode regardless of their work functions. Materialscontained in the charge-generation region will be described later.

<Hole-Transport Layer 302>

The hole-transport layer 302 contains a substance with a highhole-transport property.

The substance with a high hole-transport property is a substance with aproperty of transporting more holes than electrons, and is especiallypreferably a substance with a hole mobility of 10⁻⁶ cm²/Vs or more. Avariety of compounds can be used. For example, an aromatic aminecompound such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB or α-NPD) or4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP);a carbazole derivative such as 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA), or9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA); an aromatic hydrocarbon compound such as2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA), or9,10-diphenylanthracene (abbreviation: DPAnth); a high molecularcompound such as PVK or PVTPA.

<Light-Emitting Layer 303>

For the light-emitting layer 303, a fluorescent compound which exhibitsfluorescence or a phosphorescent compound which exhibits phosphorescencecan be used.

Examples of the fluorescent compound that can be used for thelight-emitting layer 303 includeN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), and rubrene.

Examples of the phosphorescent compound that can be used for thelight-emitting layer 303 include organometallic complexes such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III) picolinate(abbreviation: FIrpic), tris(2-phenylpyridinato-N, C²′)iridium(III)(abbreviation: Ir(ppy)₃), and(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)).

The light-emitting layer 303 may have a structure in which any of theabove-described light-emitting organic compounds (a light-emittingsubstance or a guest material) is dispersed in another substance (a hostmaterial). As the host material, a variety of kinds of materials can beused, and it is preferable to use a substance which has a lowestunoccupied molecular orbital level (LUMO level) higher than that of theguest material and has a highest occupied molecular orbital level (HOMOlevel) lower than that of the guest material.

With the structure in which the guest material is dispersed in the hostmaterial, crystallization of the light-emitting layer 303 can beinhibited. Furthermore, concentration quenching due to highconcentration of the guest material can be suppressed.

As the host material, the above-described substance with a highhole-transport property (e.g., an aromatic amine compound or a carbazolederivative) or a later-described substance with a highelectron-transport property (e.g., a metal complex having a quinolineskeleton or a benzoquinoline skeleton or a metal complex having anoxazole-based or thiazole-based ligand) can be used. As the hostmaterial, specifically, a metal complex such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq) orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq); a heterocyclic compound such as3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), orbathocuproine (abbreviation: BCP); a condensed aromatic compound such asCzPA, DNA, t-BuDNA, or DPAnth; or an aromatic amine compound such as NPBcan be used.

Alternatively, as the host material, a plurality of kinds of materialscan be used. For example, in order to suppress crystallization, asubstance such as rubrene that suppresses crystallization may be furtheradded. In addition, NPB, Alq, or the like may be further added in orderto transfer energy to the guest material more efficiently.

Further, when a plurality of light-emitting layers are provided andemission colors of the light-emitting layers are made different, lightemission of a desired color can be provided from the light-emittingelement as a whole. For example, in a light-emitting element having twolight-emitting layers, the emission colors of first and secondlight-emitting layers are complementary, so that the light-emittingelement can emit white light as a whole. Further, the same applies to alight-emitting element having three or more light-emitting layers.

<Electron-Transport Layer 304>

The electron-transport layer 304 contains a substance with a highelectron-transport property.

The substance with a high electron-transport property is an organiccompound having a property of transporting more electrons than holes,and is especially preferably a substance with an electron mobility of10⁻⁶ cm²/Vs or more.

As the substance with a high electron-transport property, for example, ametal complex having a quinoline skeleton or a benzoquinoline skeleton,such as Alq or BAlq, can be used. Alternatively, a metal complex havingan oxazole-based ligand or a thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂) orthe like can be used. Alternatively, TAZ, BPhen, BCP, or the like can beused.

<Electron-Injection Layer 305>

The electron-injection layer 305 contains a substance with a highelectron-injection property.

Examples of the substance with a high electron-injection propertyinclude alkali metals, alkaline earth metals, and compounds thereof,such as lithium, cesium, calcium, lithium fluoride, cesium fluoride,calcium fluoride, and lithium oxide. In addition, a rare earth metalcompound such as erbium fluoride can also be used. Furthermore, thesubstance for the electron-transport layer 304 described above can alsobe used.

<Charge-Generation Region>

The charge-generation region may have either a structure in which anelectron acceptor (acceptor) is added to an organic compound with a highhole-transport property or a structure in which an electron donor(donor) is added to an organic compound with a high electron-transportproperty. Alternatively, these structures may be stacked.

As examples of an organic compound with a high hole-transport property,the above materials which can be used for the hole-transport layer canbe given, and as examples of an organic compound with a highelectron-transport property, the above materials which can be used forthe electron-transport layer can be given.

Further, as examples of the electron acceptor,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F4-TCNQ), chloranil, and the like can be given. In addition, transitionmetal oxides can be given. In addition, oxides of metals that belong toGroup 4 to Group 8 of the periodic table can be given. Specifically,vanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide arepreferable since their electron-accepting property is high. Among these,molybdenum oxide is especially preferable because it is stable in theair, has a low hygroscopic property, and is easily handled.

Further, as the electron donor, it is possible to use an alkali metal,an alkaline earth metal, a rare earth metal, a metal belonging to Group13 of the periodic table, or an oxide or a carbonate thereof.Specifically, lithium, cesium, magnesium, calcium, ytterbium, indium,lithium oxide, cesium carbonate, or the like is preferably used.Alternatively, an organic compound such as tetrathianaphthacene may beused as the electron donor.

The above-described layers included in the EL layer 203 and theintermediate layer 207 can be formed by any of the following methods: anevaporation method (including a vacuum evaporation method), a transfermethod, a printing method, an inkjet method, a coating method, and thelike.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 4

In this embodiment, electronic devices which include the light-emittingdevice of one embodiment of the present invention will be described withreference to FIGS. 8A to 8C and FIGS. 9A to 9C.

The light-emitting device of one embodiment of the present invention canbe used for a flash of a camera such as a digital still camera, a flashof a camera incorporated in a mobile phone (also called a cell phone ormobile phone device) or a portable information terminal having aphotography function, or the like. Alternatively, the light-emittingdevice of one embodiment of the present invention can be used for alight of a bicycle or a car, a beacon, illuminations for decorativepurposes, or the like.

FIG. 8A illustrates an example of a digital still camera. A digitalstill camera 7300 includes a housing 7301, a lens 7304, a light-emittingdevice 7310, and the like. The light-emitting device of one embodimentof the present invention is applied to the light-emitting device 7310. Alight-emitting portion 7303 of the light-emitting device 7310 is locatedso as to surround the lens 7304. The light-emitting device of oneembodiment of the present invention is flexible and can thus be curved.In the digital still camera 7300, a non-light-emitting portion 7305 ofthe light-emitting device 7310 is bent to fit the shape of the housing7301, which enables the light-emitting portion 7303 to be located over alarge area around the lens 7304. This can make a shadow of a nose lesslikely to be cast on a cheek when a photograph of a human face is takenin a dark place using a flash, for example. Note that a light-emittingelement may be formed in the non-light-emitting portion 7305 through thesame process and may be used as an indicator of operating conditions.

FIGS. 8B and 8C illustrate an example of a mobile phone. One side (alsoreferred to as a front side) of a mobile phone 7350 is illustrated inFIG. 8B, and a side opposite the one side (also referred to as a backside) is illustrated in FIG. 8C.

The mobile phone 7350 includes a housing 7351, a display portion 7352, alens 7354, a light-emitting device 7360, and the like. Thelight-emitting device of one embodiment of the present invention isapplied to the light-emitting device 7360. The light-emitting device7360 includes a light-emitting portion 7353 and a non-light-emittingportion 7355, and the light-emitting portion 7353 is located so as tosurround the lens 7354. The light-emitting portion 7353 may be used as amirror when not emitting light.

FIG. 9A illustrates a modification example of the light-emitting device7360 of the mobile phone 7350. The light-emitting device 7360illustrated in FIG. 9A includes two light-emitting panels 7353 a and7353 b.

FIG. 10 is a block diagram of the light-emitting device 7360 illustratedin FIG. 9A. The light-emitting device 7360 includes the twolight-emitting panels 7353 a and 7353 b, a driver circuit 730, twoconstant current supplies 740 a and 740 b, and two control devices 750 aand 750 b.

The two control devices 750 a and 750 b are supplied with a signalcorresponding to a condition selected by a user of the mobile phone 7350or detection signals from a variety of sensors. Each of the controldevices 750 a and 750 b supplies a control signal in accordance with asupplied signal.

The constant current supply 740 a supplies a constant current pulse tothe light-emitting panel 7353 a in accordance with a control signalsupplied by the control device 750 a. The constant current supply 740 bsupplies a constant current pulse to the light-emitting panel 7353 b inaccordance with a control signal supplied by the control device 750 b.Thus, the amount of light of the light-emitting panels 7353 a and 7353 bis separately adjusted. As a result, the amount of light emitted fromthe light-emitting device can be adjusted in a wider range, which ispreferable.

The light-emitting panels 7353 a and 7353 b may emit light withdifferent colors or different color temperatures. For example, in thecase where the two light-emitting panels have different colortemperatures, the light-emitting device can emit light with anappropriate color temperature by separately adjusting the amount oflight of the light-emitting panels.

The driver circuit 730 includes a start switch 732. The light-emittingpanels 7353 a and 7353 b are separately supplied with a control pulsesignal by the driver circuit 730. The driver circuit 730 may supply thesame control pulse signals or different control pulse signals to thelight-emitting panels 7353 a and 7353 b.

The light-emitting device 7360 may include two or more driver circuits,and may include three or more light-emitting panels. In addition, thelight-emitting device 7360 may include a light-emitting panel whoselight amount cannot be adjusted in addition to a light-emitting panel ofone embodiment of the present invention in which the amount of light canbe adjusted.

In the light-emitting device 7360 having the structure illustrated inFIG. 10, the light-emitting panels 7353 a and 7353 b can eachindependently emit light. For example, when one light-emitting panelemits a sufficient amount of light, only one light-emitting panel ismade to emit light, and only when a larger amount of light is needed,both light-emitting panels are made to emit light. Thus, powerconsumption of the light-emitting device can be reduced anddeterioration of the light-emitting panels can be suppressed.

FIG. 9B illustrates an example of a bicycle. A bicycle 7400 includes alight 7405. The light 7405 includes the light-emitting device of oneembodiment of the present invention.

FIG. 9C illustrates an example of a vehicle. A vehicle 7410 includes alight 7415. The light-emitting device of one embodiment of the presentinvention is applied to the light 7415.

In the case where the light-emitting device of one embodiment of thepresent invention is used for a light of a bicycle or a vehicle, afterdetection of the surrounding brightness by an optical sensor, thefollowing controls can be performed, for example: when the surroundingbrightness is sufficiently high, the light does not emit light; when thesurrounding brightness is sufficiently low, the light blinks; and whenthe surrounding brightness is not sufficiently high but light isdetected, the light blinks and the amount of light emitted from thelight is increased. Thus, the light-emitting device of one embodiment ofthe present invention can emit an optimum amount of light, and a lightwith low power consumption can be achieved.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Example 1

In this example, a light-emitting panel of one embodiment of the presentinvention will be described.

A plan view of a light-emitting panel manufactured in this example isillustrated in FIG. 4A, and a cross-sectional view taken alongdashed-dotted line X1-Y1 in FIG. 4A is illustrated in FIG. 5B. Note thatsome components of the light-emitting panel are not illustrated in FIG.4A.

In the light-emitting panel of this example, as illustrated in FIG. 5B,the light-emitting element 1250 is provided over the support substrate1229 having a light extraction structure, with the insulating film 1224provided therebetween. The auxiliary wiring 1206 is provided over theinsulating film 1224 and is electrically connected to the firstelectrode 1201. A part of the auxiliary wiring 1206 is exposed tofunction as a terminal. An end portion of the first electrode 1201 andan end portion of the conductive layer 1210 are covered with thepartition wall 1205. Further, the partition wall 1205 is provided tocover the auxiliary wiring 1206 with the first electrode 1201 providedtherebetween. The light-emitting element 1250 is sealed with the supportsubstrate 1229, the sealing substrate 1228, and the sealant 1227.

In the light-emitting panel of this example, a diffusion film of apolyester-based resin was used as the support substrate 1229, and asubstrate including a thin glass layer and a polyethylene terephthalate(PET) layer was used as the sealing substrate 1228. These substrates areflexible, and the light-emitting panel of this example is a flexiblelight-emitting panel. The area of a light-emitting region of thelight-emitting panel of this example is 56 mm×42 mm.

The light-emitting element 1250 is an organic EL element having abottom-emission structure; specifically, the first electrode 1201 whichtransmits visible light is provided over the support substrate 1229, theEL layer 1202 is provided over the first electrode 1201, and the secondelectrode 1203 which reflects visible light is provided over the ELlayer 1202.

A method for manufacturing the light-emitting panel of this example willbe described.

First, a base film, a separation layer (a tungsten film), and a layer tobe separated were formed in this order over a glass substrate that was aformation substrate. In this example, the layer to be separated includesthe insulating film 1224, the auxiliary wiring 1206, the first electrode1201, and the partition wall 1205.

A total of seven auxiliary wirings 1206 were formed over the insulatingfilm 1224. At this time, the auxiliary wirings 1206 with a width L2 of322 μm were formed at a pitch of 5.3 mm. As the first electrode 1201, afilm of indium tin oxide containing silicon oxide (ITSO) was formed. Atotal of seven partition walls 1205 covering the auxiliary wirings 1206were formed to have a width L1 of 330 μm.

Then, a temporary support substrate and the first electrode 1201 wereattached using a separation adhesive. Then, the layer to be separatedwas separated from the formation substrate along the separation layer.Thus, the layer separated is provided on the temporary support substrateside.

Next, the layer which was separated from the formation substrate andwhere the insulating film 1224 was exposed was attached to the supportsubstrate 1229 using a UV curable adhesive. As the support substrate1229, a diffusion film of a polyester-based resin was used as describedabove. Then, the temporary support substrate was separated, whereby thefirst electrode 1201 was exposed over the support substrate 1229.

Next, the EL layer 1202 and the second electrode 1203 were formed overthe first electrode 1201. As the EL layer 1202, a first EL layerincluding a light-emitting layer containing a fluorescent compoundemitting blue light, an intermediate layer, and a second EL layerincluding a light-emitting layer containing a phosphorescent compoundemitting green light and a light-emitting layer containing aphosphorescent compound emitting red light were stacked in this orderfrom the first electrode 1201 side. Silver was used for the secondelectrode 1203.

Then, a photo-curable resin containing zeolite which serves as thesealant 1227 was applied and cured by UV light irradiation. Next, thesupport substrate 1229 and the substrate including the thin glass layerand the polyethylene terephthalate (PET) layer which was the sealingsubstrate 1228 were attached to each other using a UV curable adhesive.

Operation characteristics of the light-emitting panel obtained in theabove manner were measured. Voltage-luminance characteristics of thelight-emitting panel are shown in FIG. 11 as indicated by “initial” in alegend. An emission spectrum of the light-emitting panel is shown inFIG. 12. As shown in FIG. 12, it is found that the light-emitting panelof this example shows an emission spectrum including light originatingfrom the fluorescent compound emitting blue light, light originatingfrom the phosphorescent compound emitting green light, and lightoriginating from the phosphorescent compound emitting red light.

After that, a light-emitting device including the light-emitting panelwas subjected to a reliability test. In the reliability test, thelight-emitting panel was made to emit light 3000 times or 10000 timeswith intervals. For each time of light emission, a current of 2 A flowedin the light-emitting panel for 50 milliseconds (ms). The currentdensity of the light-emitting element at this time was 90 mA/cm². Theinterval between light emissions (non-light-emitting period) was 10seconds.

FIG. 11 shows voltage-luminance characteristics of the light-emittingpanel after 3000 times of light emission and those after 10000 times oflight emission.

It can be seen from FIG. 11 that the voltage-luminance characteristicsof the light-emitting panel even after 10000 times of light emissiondoes not significantly differ from those before the reliability test andthat the light-emitting panel does not deteriorate. This supports thatthe light-emitting panel of this example has high reliability.

Example 2

In this example, an organic EL element that can be applied to oneembodiment of the present invention is described.

In this example, the amount of current that can flow in an organic ELelement emitting white light was examined. The light-emitting region ofthe used organic EL element was 2 mm×2 mm. For each light emission,current was made flow in the organic EL element for 50 milliseconds(ms).

As a result, a current of 60 mA was able to flow in the organic ELelement (the corresponding current density was 1500 mA/cm²). However,when a current of 68 mA flowed (the corresponding current density was1700 mA/cm²), the organic EL element was short-circuited.

Accordingly, in the light-emitting device of one embodiment of thepresent invention which includes an organic EL element, the amount oflight can be adjusted within a range where a current density is lessthan 1700 mA/cm². Thus, a larger amount of current can flow in anorganic EL element than in a light-emitting diode or the like using aninorganic material.

This application is based on Japanese Patent Application serial no.2013-134799 filed with Japan Patent Office on Jun. 27, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a front sideincluding a display portion; and a back side a lens and a light-emittingportion surrounding the lens, wherein a constant current having a halfwidth of more than or equal to 1 millisecond and less than or equal to1000 milliseconds is configured to be supplied to the light-emittingportion, and wherein the light-emitting portion has a round shape. 2.The semiconductor device according to claim 1, wherein the semiconductordevice is a mobile phone.
 3. The semiconductor device according to claim1, wherein the light-emitting portion comprises a light-emitting elementcomprising a first electrode, a second electrode overlapping with thefirst electrode, and an EL layer between the first electrode and thesecond electrode.
 4. The semiconductor device according to claim 1,wherein the lens and the light-emitting portion are configured tofunction as a camera.
 5. A semiconductor device comprising: a front sideincluding a display portion; and a back side a lens and a light-emittingportion surrounding the lens, wherein a constant current having a halfwidth of more than or equal to 1 millisecond and less than or equal to1000 milliseconds is configured to be supplied to the light-emittingportion, and wherein a current density of a light-emitting element inthe light-emitting portion is configured to be greater than or equal to10 mA/cm² and less than or equal to 1000 mA/cm².
 6. The semiconductordevice according to claim 5, wherein the semiconductor device is amobile phone.
 7. The semiconductor device according to claim 5, whereinthe light-emitting portion comprises the light-emitting elementcomprising a first electrode, a second electrode overlapping with thefirst electrode, and an EL layer between the first electrode and thesecond electrode.
 8. The semiconductor device according to claim 5,wherein the lens and the light-emitting portion are configured tofunction as a camera.
 9. A semiconductor device comprising: a front sideincluding a display portion; a back side a lens and a light-emittingportion surrounding the lens, and an optical sensor configured to detecta brightness around the semiconductor device, wherein a constant currenthaving a half width of more than or equal to 1 millisecond and less thanor equal to 1000 milliseconds is configured to be supplied to thelight-emitting portion, and wherein an amount of light emitted from thelight-emitting portion is configured to be adjusted by the detectedbrightness.
 10. The semiconductor device according to claim 9, whereinthe semiconductor device is a mobile phone.
 11. The semiconductor deviceaccording to claim 9, wherein the light-emitting portion comprises alight-emitting element comprising a first electrode, a second electrodeoverlapping with the first electrode, and an EL layer between the firstelectrode and the second electrode.
 12. The semiconductor deviceaccording to claim 9, wherein the lens and the light-emitting portionare configured to function as a camera.